This invention relates to a liquid level detecting device which, when used in tanks and storage means for holding gasoline, oil and various other liquids, determines the presence of such liquids at prescribed positions inside such containers. More particularly, this invention relates to a liquid level detecting device of the type which serves as detection means for alarm systems adapted to detect and issue warnings when the liquid supplies in such containers are running short or for liquid volume measuring systems adapted to detect heights of liquid columns held in such containers and which further possesses a function of digital display of the results of detection.
Conventional the liquid level detecting devices serving the same purpose as that of this invention have included a device which, upon installation in a fuel tank of, for example, an automobile, detects that the fuel supply in the tank is running short and issues a proper warning to the driver such as by means of a pilot lamp. The most popular mechanism adopted for devices of this type makes effective use of a float which is attached to the free end of an arm freely rotatably attached at the other end to a pivot disposed near the bottom of the tank, whereby the float remains in a raised position when the tank is relatively full of fuel falls to a lower position as the fuel supply becomes insufficient and, as the float keeps on falling and eventually reaches a prescribed position, the terminal disposed on the underside of the float comes into contact with an opposed terminal to close a circuit and light a pilot lamp.
Such a mechanical detecting device is unreliable. Since the mechanism is completely immersed in the fuel, it undergoes accelerated deterioration and tends to incur troubles due to failure of contact. If the terminals are positioned in the space above the liquid surface and, consequently, they induce the phenomenon of chattering (repeated cycles of making and breaking contact) because of external shocks such as the vibration of the tank proper, there may possibly ensue erroneous detection and there is a possibility of the applied potential giving rise to arcs between the terminals and setting fire to the inflammable liquid as well.
For the purpose of overcoming such disadvantages of the use of mechanical means, there has been proposed a detection system designed to electrically determine the presence or absence of a liquid at a prescribed position. Such a conventional electrical detection system, however, makes use of an element in which the magnitude of electric current flowing therethrough varies when the element is enclosed with the liquid and when it is not. This variation is so slight that, for the variation in the electric current to actuate switching means of a pilot lamp effectively, there must be provided a suitable amplifier. Besides, effective conduction from the point of detection to the amplifier inevitably requires adoption of a lead wire free from appreciable loss of current and, consequently, is expensive. This system is not very economical and is not completely satisfactory in terms of circuit configuration. Furthermore since the variation in the electrical magnitude utilized for the detection is very small, noises and other similar disturbing factors finding access to the circuit from external sources frequently cause erroneous detection.
An object of this invention is to provide a liquid level detecting device which avoids the inherent disadvantages suffered by mechanical detection means and enjoys high sensitivity and accuracy and which can be manufactured inexpensively and compactly.
Another object of this invention is to provide a liquid level detecting device which can be utilized not merely as detection means for an alarm system adapted to indicate a very small balance of the liquid in the container but also as detection means in a liquid volume measuring system serving to display digitally the liquid level or liquid volume in the container.